1. Field of the Invention
The present invention relates to a memory circuit in which the logic state is not erased after the power is turned off, to a memory unit and a signal processing circuit using the memory circuit, to a method for driving the memory circuit, the memory unit, and the signal processing circuit, and to an electronic device using the signal processing circuit.
2. Description of the Related Art
Signal processing circuits such as central processing units (CPUs) vary in structure depending on the intended use. A signal processing circuit generally has a main memory for storing data or program and other memory units such as a register and a cache memory. A register has a function of temporarily holding data for carrying out arithmetic processing, holding a program execution state, or the like. Meanwhile, a cache memory, which is located between an arithmetic circuit and a main memory, is provided to reduce low-speed access to the main memory and speed up the arithmetic processing.
In a memory unit, such as a register or a cache memory, data writing needs to be performed at higher speed than in a main memory. For this reason, in general, a flip-flop circuit or the like is used as a register, while a static random access memory (SRAM) or the like is used as a cache memory. In other words, such a register, a cache memory, or the like is a volatile memory unit which loses data after the supply of power is stopped.
In order to achieve low power consumption, a method in which the supply of the supply voltage to a signal processing circuit is temporarily stopped while input/output of data is not conducted has been suggested. In the method, a nonvolatile memory unit is located in the periphery of a volatile memory unit such as a register or a cache memory, and the data is temporarily stored in the nonvolatile memory unit. Thus, the register, the cache memory, or the like in the signal processing circuit holds data even while the supply of the supply voltage is stopped (see Patent Document 1, for example).
In the case where the supply of the supply voltage to a signal processing circuit is stopped for a long time, data in a volatile memory unit is transferred to an external memory unit such as a hard disk or a flash memory before the supply of the supply voltage is stopped, so that the data can be prevented from being lost.